1) Filed of the Invention
The present invention relates to a turbo molecular pump which is used as a vacuum apparatus of a semiconductor manufacturing apparatus or the like, and in particular to a turbo molecular pump having improved exhaust performance and high reliability.
2) Description of the Related Art
FIG. 19 shows an entire arrangement of a turbo molecular pump. In FIG. 19, a rotor 1 is axially floated by an axial electromagnet 3, and the position of the rotor 1 in a radial direction is controlled by radial electromagnets 5A and 5B. The rotor 1 is rotated by a motor 7. The rotor 1 is formed with rotor blades 9 arranged in multiple stages in the axial direction. A plurality of stator blades 11 are disposed in multiple stages to define clearances from rotor blades 9. The end of the each stator blade 11 is supported by and between a plurality of spacers 13 that are stacked one on another and integrally connected to a housing 20.
FIG. 20 shows an external view of the stator blade 11. The turbo molecular pump thus constructed performs the exhaust action in such a manner that the rotating rotor blades 9 beat gaseous molecule to move the same in the axial direction. The turbo molecular pump of this type is used, for instance, to exhaust the gas in a chamber of a semiconductor manufacturing apparatus. That is, the gas, which is always supplied to the chamber for processing of the semiconductor, is discharged therefrom by the turbo molecular pump.
FIG. 21 is an enlarged view of a portion bracketed by a dotted line in FIG. 19. In the illustrated turbo molecular pump, a clearance between the rotor blade 9 and the stator blade 11 is constant along the entire length of the rotor blade 9. In order to determine this clearance xcex4, a tolerance regarding a clearance between a protective ball bearing 15 and the rotor 1, the machining accuracy and assembling accuracy of the components, etc. must be taken into account. Further, the flexure of the rotor blade 9 and the stator blade 11, which is caused by the introduction of the atmospheric air under the pump operation, the externally-applied impact, the touch-down caused, for instance, by the current-cut-off, etc., must also be taken into account.
Upon the consideration of the above factors, the clearance xcex4 must be determined to be such a dimension as to keep the rotary blade 9 and the stator blade 11 in non-contact with each other. The flexure of the rotor blade 9 is a primary factor (about 30%) to be considered for determining the clearance xcex4.
On the other hand, this clearance xcex4 is closely related to the exhaust performance of the turbo molecular pump. Recent tendency is to increase the amount of the gas supplied to the chamber, and therefore the amount of the gas to be exhausted under the normal operation of the turbo molecular pump is being increased. FIG. 22 shows a test result of flow quantity characteristic.
In FIG. 22, Ps is defined as a pressure at which the gas is changed from a molecular flow region to an intermediate flow region. The hatched portion in the drawing shows a degree of lowering of the performance. It can be found out from this test result that a sufficient exhaust performance was obtained in the molecular flow region in which the flow quantity of the gas to be discharged was relatively low, but the sufficient exhaust performance was not obtained if the flow quantity of the gas to be discharged is increased to reach the intermediate flow region.
The lowering of the exhaust performance in association with the increase of the gas flow quantity is in correlation with the size of the clearance xcex4 existing between the rotor blade 9 and the stator blade 11. Whether or not the gas in the clearance between the rotor blade 9 and the stator blade 11 is handled as the molecular flow region depends on the mean free path of the gas, and this mean free path is expressed approximately by Formula 2.
[Formula 2]                              Ps          ⁡                      (            Torr            )                          =                  0.05                      δ            ⁢                          xe2x80x83                        ⁢                          (              mm              )                                                          [                  Formula          ⁢                      xe2x80x83                    ⁢          2                ]            
That is, as the clearance xcex4 becomes smaller, the start pressure Ps of the intermediate flow region becomes higher. If the start pressure Ps becomes higher, it is possible to avoid the lowering of the performance accordingly even if the flow quantity Q becomes larger.
It is, however, noted that a certain flexure is inevitably caused on the rotor blade 9 and the stator blade 11 in the case where the introduction of the atmospheric air under the pump operation, the externally-applied impact, the touch-down caused, for instance, by the current-cut-off, etc. occur. For this reason, simply making the clearance xcex4 smaller gives rise to the likelihood of the contact between the rotor blade 9 and the stator blade 11.
The present invention was made in view of the above-noted problem encountered in the related turbo molecular pump, and an object of the present invention is to provide a turbo molecular pump which has an improved exhaust performance with its reliability maintained.
A turbo molecular pump according to the present invention comprises: a rotor having rotor blades arranged in multiple stages; and stator blades arranged in multiple stages, the rotor blades and the stator blades being alternately arranged in an axial direction with spaces therebetween, which is characterized in that a spatial clearance between a proximal end of a rotor blade and a stator blade adjacent thereto is made smaller than a spatial clearance between a distal end of the rotor blade and the stator blade.
The spatial clearance between the proximal end of the rotor blade and the distal end of the stator blade adjacent to the rotor blade is made smaller than the spatial clearance between the distal end of the rotor blade and the proximal end of the stator blade. Here, the proximal end of the rotor blade means a fixed side of the rotor blade to the rotor, i.e. an inner circumferential side. The proximal end of the stator blade means the outer circumferential side of the stator blade supported between a plurality of stator vane spacers that are stacked one on another.
Making the spatial clearance between the rotor blade and the stator blade smaller effectively enhances the exhaust performance of the turbo molecular pump. It is, however, noted that making the spatial clearance smaller results in the liability that the rotor blade may contact the stator blade. For this reason, the spatial clearance on the proximal end side of the rotor blade, which is less flexured, is set to be smaller in comparison to the spatial clearance on the distal end side of the rotor blade, which is more flexured. This arrangement makes it possible to avoid the lowering of the performance even if the flow quantity becomes larger.
The turbo molecular pump of the present invention is characterized in that at least one of upper and lower surfaces of the rotor blade and/or at least one of upper and lower surfaces of the stator blade is/are contoured to present a flexure curve line expressed by Formula 1 or approximately expressed by Formula 1 where Young""s modulus of material is E (kgf/mm2), a geometrical moment of inertia of a beam is I (mm4), an entire length of the beam is L (mm), a distributed load on the beam is W (kgf/mm), and a flexure amount of the beam at a distance x (mm) from an open end of the beam is xcex94 (mm), a thickness of the rotor blade in the axial direction is thinner as it approaches the distal end of the rotor blade, and a thickness of the stator blade in the axial direction is thinner as it approaches a proximal end of the stator blade.
[Formula 1]                    Δ        =                                            W              ⁢                              xe2x80x83                            ⁢                              L                4                                                    8              ⁢                              xe2x80x83                            ⁢              E              ⁢                              xe2x80x83                            ⁢              I                                ⁢                      (                          1              -                                                4                  ⁢                  X                                                  3                  ⁢                  L                                            +                                                X                  4                                                  3                  ⁢                                      L                    4                                                                        )                                              [                  Formula          ⁢                      xe2x80x83                    ⁢          1                ]            
The contour may be a curved configuration or otherwise may be a linear configuration, i.e. a tapered configuration. The tapered configuration can be obtained only by a plano-processing, so that it is easily available in comparison with a case in which the curved configuration is obtained by processing. Formula 1 expresses the flexure amount or deflection at an arbitrary point on the simple cantilever beam. At least one of the upper and lower surfaces of the rotor blade and/or at least one of the upper and lower surfaces of the stator blade is/are processed along the flexure curve line expressed by Formula 1 or approximately expressed by Formula 1. This makes it possible to design the spatial distance between the rotor blade and the stator blade as optimum and continuous amounts along the flexure curve line consequently, the pressure from which the intermediate flow region begins can be set as high as possible.
The turbo molecular pump of the present invention is characterized in that at least one of upper and lower surfaces of the rotor blade and/or at least one of upper and lower surfaces of the stator blade is/are provided in a stepwise manner with at least one step along a flexure curve line expressed by a formula 1 or approximately expressed by Formula 1, a thickness of the rotor blade in the axial direction is thinner in a stepwise manner as it approaches the distal end of the rotor blade, and a thickness of the stator blade in the axial direction is thinner in a stepwise manner as it approaches a proximal end of the stator blade.
At least one step is provided in the stepwise manner along the flexure curve line expressed by Formula 1 or approximately expressed by Formula 1. By providing the step or steps in the stepwise manner, the spatial clearance between the rotor blade and the stator blade can be set in the stepwise manner to make it difficult to contact the rotor blade with the stator blade. It can be formed only by the plano-processing which is easier than the curved processing.
The turbo molecular pump of the present invention is characterized in that a single step is provided at a position in a range of 60-85% from the proximal end of the rotor blade or the distal end of the stator blade.
The only one step is provided. The amount of the work as to the exhaust action of the turbo molecular pump is evaluated in order to determine an effective position at which the step should be provided on the blade. To this end, the evaluation is carried out on a certain value, which has a correlation with this amount of the work, on the basis of the clearance between the rotor blade and the stator blade obtained from the flexure curve line expressed by Formula 1 or approximately expressed by Formula 1.
The pressure, from which the lowering of the exhaust performance in the turbo molecular pump initially occurs, depends on the clearance between the rotor blade and the stator blade, and increases in an inverse-proportional manner as the clearance becomes smaller. It can be found out from the result of the evaluation that the step is disposed at a position in a range of 60-85% from the proximal end of the rotor blade or the distal end of the stator blade to enhance the exhaust performance. The provision of the single step makes the processing easy.